Conventionally, rotary machinery in such forms as electric generators and electric motors is sometimes constructed as a slotless type having no slot for receiving coils to the end of reducing torque ripples (or cogging torque). See Japanese patent laid open publication No. 2002-272049A, for instance. In the slotless motor described in this prior patent publication, a flat conductor having a rectangular cross section is used instead of a more common conductor having a circular cross section to the end of maximizing the space factor. The flat wire is wound in a spiral path from the inner circumference to the outer circumference. On the other hand, Japanese patent laid open publication No. 2002-247791A discloses a coil winding method which is suitable for winding hexagonal coils for slotless motors having a core although hexagonal coils are typically used in coreless motors. In a hexagonal coil, each turn has an identical, substantially hexagonal shape, and is slightly shifted from the adjacent turn with a suitable amount of overlap along the circumferential direction (which is sometimes called as distributed winding) so that a coil having the shape of a thin cylindrical shell may be formed. Japanese patent laid open publication No. 2002-247791A also shows lozenge-shaped coils and honeycomb coils for use in coreless motors in addition to the hexagonal coil.
FIGS. 1 to 3 show the structure of a typical slotless permanent magnet generator. FIG. 1 is a schematic exploded perspective view of the slotless permanent magnet generator. FIG. 2 is a longitudinal schematic sectional view. FIG. 3 is a sectional view taken along line III-III of FIG. 2. As shown in these drawings, the slotless permanent magnet generator 1 comprises a substantially cylindrical shaft (rotor) 3 having a permanent magnet 2 incorporated therein, a stator core 4 surrounding the rotor 3 and a coil assembly 5 fixedly attached to the inner circumferential surface of the stator core 4 so as to define an air gap with respect to the outer circumferential surface of the rotor 3. The rotor 3 is rotatably supported by bearings not shown in the drawings, and an electric voltage is induced in the coil assembly 5 as the rotor 3 turns. The coil assembly 5 includes three independent coils so as to generate the voltage in three phases (U, V and W) which are electrically 120 degrees apart. Each coil has a plurality of turns which are each offset from the adjacent one in the direction of rotation so that the coil as a whole extends in the circumferential direction. In this example, each turn is lozenge-shaped. Such a slotless permanent magnet generator 1 is suited to be used as a small generator, but the rotor 3 is required to be rotated at high speed to obtain a large output.
FIG. 4a is a fragmentary enlarged view of FIG. 3, and shows that the coil wire of each coil consists of a conductor 10 having a square cross section. The conductor 10 is covered by electrically insulating material although it is not shown in the drawing. The conductor 10 is wound in two layers, an inner layer and an outer layer, in the cross sectional view of FIG. 4a but it only means that the coil is wound so that the adjacent turns overlap each other and the conductor for the coil of each phase consists of a single length of coil wire. In case of a two pole generator in which the permanent magnet 2 of the rotor 3 has only one N pole and one S pole, there may be two coils for each phase which are separated from each other by 180 degrees around the rotor 3, and connected to each other in series by a connecting wire. In any case, when each coil consists of a single conductor 10 having a square cross section, the copper loss can be reduced by increasing the cross sectional area of the conductor 10. However, when the cross sectional area of the conductor 10 is increased, the eddy current loss caused by the electric current that flows in the cross sectional plane in the manner of vortices rapidly increases with the increase in the rotational speed of the rotor 3 (typically to the 1.6 to 1.8th power of the rotational speed of the rotor 3) as shown in the graph of FIG. 4b. This seriously reduces the efficiency of the generator in a high speed range.
In general, the eddy current diminishes as the cross sectional area of the conductor 10 gets smaller. Therefore, it is conceivable to form the coil by using conductors 10a each having a smaller (for instance ¼), rectangular cross section as illustrated in FIG. 5a. In such a case, to minimize the increase in copper loss due to the reduction in the cross sectional area of the conductors 10a, it is common to form two coil segments for each phase and connect the two coil segments in parallel to each other. By so doing, the increase of the eddy current loss with the rise in the rotational speed of the rotor 3 can be controlled. However, as the rotational speed of the rotor 3 increases, a difference arises between the electromotive force between the two coil segments that are connected in parallel to each other, and the resulting circulating flow of electric current between the two coil segments gives rise to circulating current loss. This prevents the reduction in loss in a high speed range. If only one coil segment is used for each phase to avoid circulating current loss, the reduction in the cross sectional area of the conductor increases the copper loss to a significant extent.
It is known to wind a flat conductor having a rectangular cross section in an edgewise manner in choke coils or the likes (see Japanese patent laid open publication No. 2002-203438A). It is also known to use a Litz conductor having a flat rectangular cross section in induction heating coils (see Japanese patent laid open publication No. 2000-215972A).